Why I Always Check My Wet Bulb Temperature Before a Summer Run
Exercising in the heat can be downright unpleasant or downright dangerous, depending on the weather, and there’s a great way to tell which day it is. This summer, I’ve been checking my wet-bulb temperature before almost every run. This is more important to me than the actual temperature or even the “feels like” temperature because it helps me assess the safety of an outdoor workout. I’ll postpone or cancel a workout if the wet-bulb temperature is too high.
I always knew that running in the heat of summer was harder, but this year I experimented with the Core body temperature sensor to study my body’s response to heat. (It’s a cool gadget — here’s my review of it .) In the spring, I sought out warmer weather to better adapt my body to running in hot weather. But here we are, mid-July, and even my hard-won heat adaptation isn’t enough to keep me warm during the hottest, most humid days of the heat wave.
To help you stay safe while exercising during the hottest time of year, let me explain what wet bulb temperature means and what numbers to look for when deciding whether or not to exercise outdoors.
It’s the heat and humidity.
It is often safe to exercise in the heat, and exercising in the heat forces the body to adapt to it . However, our bodies have their limits. There are temperatures and conditions that make prolonged exercise dangerous. The important thing is that it is not just the temperature. Humidity matters, too. A hot, dry day is much easier for the body to tolerate than a hot, humid one.
Below, I’ll explain how to use wet-bulb temperatures to determine when the combination of heat and humidity makes exercise dangerous. But here are a few examples: If you try to avoid wet-bulb temperatures above 82 degrees (a common recommendation), all of these scenarios would be “too hot” for a long or intense run:
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80 degrees will be too hot with 70% humidity.
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90 degrees will be too hot with 30% humidity.
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100 degrees will be too hot with 10% humidity.
If you’re used to the heat, you’ll be able to train in higher temperatures, as we’ll see. But first, let’s look at what “wet bulb temperature” even means.
How to Use a Wet Bulb Thermometer Temperature
Instead of simply looking at the day’s temperature forecast, the best way to understand how the heat will affect your body is to look at a number called the wet bulb temperature (WBGT). This number is based on the idea that if you wrap the bulb of an old thermometer in a damp cloth, the thermometer will give you a reading that tells you how much the air can cool you down. (I’m simplifying; the process is more complicated and requires three thermometers .)
This is important because one of our body’s primary cooling mechanisms, sweating, cannot function effectively when humidity is too high. Sweat cools us by evaporating from the skin, but humidity inhibits this process. The WBGT index measures how effectively evaporation cools a wet surface, in addition to taking into account the temperature itself.
Luckily, you don’t need a thermometer and a wet cloth to calculate the wet-bulb temperature: you can get an approximation by looking at the temperature and humidity on a chart, or do as I do and use the Carrot Weather app to see it in the forecast. WeatherFX ( iPhone , Android ) is another app that gives you the wet-bulb temperature directly.
With Carrot, I can see my WBGT every hour of the day. Interestingly, the humidity is usually higher during cooler times of the day, so the wet bulb temperature doesn’t always change much from morning to evening. I still prefer evening runs when possible, since the sun is often above the horizon and not shining directly on me. But I can use the daytime wet bulb temperature to decide whether to cut my run short or skip it altogether.
How to adjust your workout based on your wet bulb temperature
Now that you have your WBGT, you need to decide what to do about it. A map of the United States divides the country into three regions, each with a different deadline for canceling training. (If you live in Texas, you’re expected to be able to handle slightly more heat than in Minnesota.) Similar guidelines apply to road race organizers (like marathons). If the WBGT exceeds 82 degrees, the race should be canceled. Above 73 degrees, the recommendation is to “use extreme caution and slow down.”
To give an idea of what this might look like, a WBGT temperature of 82 degrees could occur at 75 degrees and 90% humidity, at 84 degrees and 50% humidity, or at 100 degrees and 10% humidity.
If you are not used to the heat
The American College of Sports Medicine offers the following general recommendations for those not accustomed to exercising in the heat:
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If your body temperature is above 65 degrees Fahrenheit , try to get more rest and watch your fluid intake.
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If your WBGT is above 72 , take extra rest and limit your exercise duration.
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If your WBGT is above 78 , take extra rest, limit the duration, and reduce the intensity (for example, don’t try to run too fast).
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If the WBGT index is above 82 , you should rest as much as you work, doing all of the above and watching closely for signs of heat stroke, as the risk is high here.
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If WBGT is above 86 , stop and go home.
If you have already adapted to the heat
For people who have acclimatized to the heat, i.e. have been safely exercising in the heat for at least a few weeks, the safety rules can be relaxed a little, since their body has already learned to cool itself better. In such cases, the threshold values are as follows:
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If your WBGT is above 72 , you can continue to exercise as usual, but pay special attention to hydration.
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If the WBGT is above 82 , “plan intense or prolonged physical activity with caution” and watch for signs of heat stroke in people at high risk.
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If the WBGT index is above 86 , limit strenuous exercise and exposure to hot weather. Watch for signs of heat stroke.
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If WBGT is above 90 , the risk is too great even for acclimatized athletes.
How Wet Bulb Temperature Affects My Real-World Training
Personally, I view anything between 62 and 72 degrees as “sucks, but it’s summer.” Most of my outdoor runs are in these conditions. When the temperature gets above 72 degrees, I become especially careful about taking precautions. I avoid direct sunlight (opt for shady trails or evening runs) and make sure to bring plenty of water. For me, this means carrying at least a liter (16 ounces) in a hydration belt for an hour-long run. If I’m driving to my route, I also make sure to have plenty of water in the car in case I get thirsty when I get back.
When the wet-bulb temperature is around 75 degrees, I do all of the above, but even more: If it’s a long run, I plan to get back to the car or home after the first 3-5 miles to rest a bit in the air conditioning. Yesterday, I ran 10 miles on trails, split into two 5-mile sections, with a quick stop in the car in the middle. (I packed a sandwich to snack on during the break, and reapplied sunscreen before heading out.)
It’s important to note that I’ve found that running speed plays a big role. An easy jog in 22 degrees wet bulb is no big deal. But if I’m planning on doing something more challenging, like a tempo run or a workout with lots of high intensity intervals, I need to be prepared to cut it short if the heat gets unbearable.
So far, I haven’t been outside when the wet-bulb temperature has been 80 degrees or higher. On the few days when it’s been that high, I’ve been able to reschedule my run for another day. Flexibility is a great thing when it comes to summer training.
All of these tips are in addition to the usual precautions for staying safe in the heat. Bring plenty of fluids and electrolytes to stay hydrated, and be sure to find shade or air-conditioned areas to cool off. (A kiddie pool with a bag of ice is one option the U.S. Soccer Association recommends.)
It’s also important to know the signs and symptoms of heat exhaustion and heat stroke , and what to do if you notice them in yourself, a teammate, or a training partner. Heat stroke is a 911-level emergency, so seek help if needed.